Our proposed studies will investigate the potential of regulatory T cells (Treg) for the promotion of tolerance to heterotopic heart allografts in cynomolgus monkeys. There is persuasive evidence, based on small animal studies, that depletion of T effector cells prior to the administration of Treg is advantageous to the outcome of Treg-induced immunomodulation. We have developed a T cell-depleting regimen in Indonesian cynomolgus monkeys, using humanized anti-CD52 mAb (alemtuzumab) in combination with mycophenolate mofetil (MMF). CD3[+]T cells can be maintained at very low numbers for periods >4 weeks, and CD4[+] T cells for several months. We have also isolated and successfully expanded monkey Treg and demonstrated their immunomodulatory properties. We hypothesize that posttransplant adoptive transfer of autologous Treg to lymphocyte-depleted hosts, in combination with a course of rapamycin monotherapy, will enhance heart allograft survival and promote tolerance induction. Our proposed investigations have the following Aims: Aim I will determine the Influence of alemtuzumab on heart allograft outcome and host immune reactivity in cynomolgus monkeys; Aim II will assess the impact of polyclonal Treg (either expanded naturally-occurring Treg or induced Treg) on heart allograft outcome and underlying mechanisms in alemtuzumab-treated monkeys and will compare the results with those obtained in Aim I; Aim III will assess the ability of alloantigen-specific Treg to promote heart allograft outcome and underlying mechanisms in alemtuzumab-treated monkeys treated as in Aim I and will compare the results with those in Aims I and II. In each Aim, the Treg will be administered on day 21, after the initial T cell depletion and heart transplantation (day 0); immunosuppressive therapy will be tapered and discontinued at 3 months. Follow-up will be for 6 months post transplant. Outcomes will be monitored by (i) length of graft survival, (ii) incidence of graft vasculopathy (chronic rejection), (iii) development of anti-donor T cell responses, (iv) development of anti-donor antibodies (Abs), (v) development of anti-vimentin and anti-myosin Abs, and T cell responses to these autoantigens, and (vi) assessment of memory T cell responses. In addition, in Aims II and III, we will ascertain the mechanistic basis of the influence of the Treg therapy on host immune reactivity and transplant outcome. These studies will provide definitive information on the ability of adoptively-transferred autologous Treg to induce a state of tolerance or prope tolerance to non-human primate heart allografts, and may provide sufficient data to justify a clinical trial.